Conventionally, several techniques have been proposed to cause an image processing apparatus to suppress jaggies generated in a low-resolution printer.
FIG. 14 indicates jaggies generated in, e.g., the edge portion of a character.
FIG. 14 is a schematic view for explaining an example of a character image in a conventional image processing apparatus. FIG. 14 shows a state wherein jaggies are generated in, e.g., the edge portions of a character.
Referring to FIG. 14, each cell corresponds to one pixel. In this example, the density is 300 dpi. An “L” shaped portion indicates a jaggy. Fifteen “L” shaped portions are shown in FIG. 14.
In a conventional technique to suppress the jaggy, pattern matching is executed for jaggies. Pixel is added to each portion corresponding to the pattern, or pixel that causes the jaggy is removed, thereby suppressing the jaggies.
The pattern matching is a known technique, and a detailed description thereof will be omitted.
FIGS. 15A and 15B depict schematic views for explaining an example of character processing in the conventional image processing apparatus. FIGS. 15A and 15B show states wherein jaggies generated in, e.g., the edge portions of a character are suppressed.
In the state shown in FIG. 15A, halftone data is added to each pixel corresponding to the pattern. Note that halftone data is added to each “L” shaped portion described above. Especially in a printer with an electrophotographic process, the image quality can be improved greatly only by adding halftone data to jaggy portions.
In the state shown in FIG. 15B, pixel divided data is added to each pixel corresponding to the pattern.
“Pixel division” is a technique of dividing one pixel of 300 dpi into a plurality of parts and a dot is assigned to each part. In this example, a pixel is vertically divided into two parts. The pixel division is also a known technique, and a detailed description thereof will be omitted.
As described above, an example of smoothing processing is implemented by adding a halftone dot or pixel divided dot to each jaggy portion. As another possible processing, dark pixels in jaggy portions may be changed to light dots, i.e. halftone dots, or full dots in jaggy portions are converted into pixel divided dots.
Japanese Patent Laid Open No. 10 42141 discloses a technique of improving such problems in a conventional image processing apparatus of this type.
A technique shown in FIGS. 16A and 16B is also known. This technique is used to suppress jaggies generated by halftoning processing such as screen processing. The jaggies are generated at a resolution lower than that of a printer. The jaggies were not suppressed by the above described method, but are suppressed by the technique disclosed in JPA 10 42141.
More specifically, the jaggy portions shown in FIG. 16A are generated by an image transforming method such as screen processing. The image quality becomes poor at a resolution lower than the printer resolution (300 dpi in this example) as shown in FIG. 16A. In FIG. 16B, halftone dots (halftone data) are output to each edge portion, thereby suppressing the jaggies.
As the printer resolution rises, the above described jaggies caused by the printer resolution rarely pose problems. More specifically, when the printer resolution increases to, e.g. 600 dpi or 1,200 dpi, jaggies are rarely recognized by human eyes.
However, jaggies generated by the image transforming method such as screen processing can easily be recognized even at a higher printer resolution. In screen processing, pixels are formed by concentrating a plurality of dots by using a pattern generally called fattening, thereby expressing a pseudo screen resolution (number of lines). Hence, a screen resolution (number of lines) higher than the printer resolution is never used. When screen processed image data is output by a printer having a resolution of, e.g., 600 dpi, the screen resolution (number of lines) is generally, about 133 to 175 lines and, at most, about 268 lines. If a screen with a larger number of lines than the above number of lines is used, no stable image quality can be obtained because of the characteristic of the electrophotographic printer. The unit of the number of lines is generally defined as LPI (Lines Per Inch).
To suppress jaggies generated by the latter image transforming method (screen processing), a method of assigning halftone dots to the above described edge portions is used. However, this method requires complex operations and a large memory capacity. For this reason, an apparatus equipped with the technique to implement the method tends to be expensive. To equip an inexpensive electrophotographic printer or MFP with the technique, a method capable of suppress the above described jaggies with a simpler arrangement is necessary.